1. Field of the Invention
The present invention relates to methods of improving productivity of crop plants. In particular it relates to the application of compositions comprising auxins to plants.
2. Background Art
Plant hormones play an important role in controlling plant growth and development. One class of plant hormone, auxins, are known to affect a number of plant processes, such as cell division, shoot elongation, phototropic responses, and apical dominance.
Soils are known to contain compounds with auxin-like activities. These compounds are thought to be derived from microbial metabolism of various substrates. For instance, L-tryptophan has been reported to serve as precursor for the microbial formation of indole-3-acetic acid (IAA), one of the major auxin products found in soils (see, e.g., Arshad and Frankenberger Plant Soil 133:1-8 (1991)).
The synthesis of IAA upon application of tryptophan to soil has been shown to effect plant growth. For instance, growth of Douglas fir was increased by application of tryptophan and inoculation with a fungus capable of producing IAA from tryptophan (Frankenberger and Poth Appl. Environ. Microbiol 53:2908-2913 (1987)). In addition, soil-applied tryptophan as well as auxins were shown to increase growth of radish (Frankenberger et al. Plant Soil 129:235-242 (1990). An investigation of the influence of various levels of tryptophan and auxins such as IAA, indole-3-acetamide (IAM), and tryptophol revealed that these compounds had distinct effects on the growth of corn. In particular, the growth parameters studied there were found not to be influenced by IAM (Sarwar and Frankenberger Plant and Soil 160:97-104 (1994)).
The metabolism of tryptophan in soil is complex. Metabolism of tryptophan results in the formation of many products including niacin and serotonin, in addition to auxins, and is regulated by a number of microbial enzymes (see, Martens and Frankenberger Soil Biol. Biochem. 25:1679-1687 (1993) and FIG. 1). Recent studies indicate that the soil microbiota has more influence on auxin production than the physicochemical properties of the soil (Sarwar et al. Plant and Soil 147:207-215 (1992)).
A need exists for inexpensive, effective means for improving crop productivity. Means for improving productivity by providing auxins to plants in a readily available form would be particularly desirable. The present invention addresses these and other needs.